<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

        /* 嵌套元素的外边距合并 */
        /*
         1.给父亲加一个边框 
         2.给父亲一个内边距也可以
         3.给父亲加一个overflow:hidden

        */
        .father{
            width: 400px;
            height: 400px;
            background-color: green;
            margin-top: 100px;
            /* border: 1px solid red; */
            /* padding-top: 1px; */
            overflow: hidden;
        }
        .son{
            width: 200px;
            height: 200px;
            background-color:purple;
            margin-top: 20px;
        }
    </style>
</head>
<body>
    
    <div class="father">
        <div class="son"></div>
    </div>    
</body>
</html>